While the amount could change, buying $60 billion in Treasury bills over a month is substantial, even by the Fed's standards. For context, the Fed bought about $85 billion in bonds each month during its final round of quantitative easing, which started in 2012.

Yet the new purchases are different from those postcrisis packages.

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Mr. Powell and his colleagues have repeated, time and again, that the current balance sheet expansion should not be confused with quantitative easing.

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To drive home the point that there is no broader policy signal this time, officials made the new package look unique. The Fed is buying only Treasury bills, for one thing. The Fed's recession-era buying focused on bonds, in a bid to make mortgages and car loans cheaper by pushing down longer-term interest rates. By concentrating this effort on short-term debt, the Fed is forgoing that sort of stimulus.

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Still, some onlookers were skeptical that the Fed would manage to convince investors that this was not an attempt to bolster the economy, given the size of the purchases.

"When it swims like a duck and quacks like a duck, it's hard to prove your intentions aren't fowl," Paul Ashworth, chief economist at Capital Economics, wrote in research note.

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It is important that the Fed's message sticks. The Fed will be short on room to cut interest rates when the next recession hits, because they are already at just 2 percent -- leaving the central bank with far less than the five percentage points of cuts it made in the 2007 to 2009 downturn. That means bond-buying will be an essential part of the Fed's future easing packages, and one to be used in case of emergency.

"They want to keep Q.E. as something special," said Laura Rosner, a co-founder of MacroPolicy Perspectives. "I don't think they want to send a signal that things are bad."

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Officials had decided this year that they wanted to continue setting interest rates in what they called an "ample reserve" framework. In such an approach, the central bank keeps its balance sheet holdings big enough to leave plenty of cash in the financial system. Banks keep their extra cash on deposit at the central bank, and the Fed adjusts interest rates by changing how it pays on those excess holdings, commonly called reserves.